Oakland and federal law enforcement officials confiscated more than four dozen firearms in a major gun and narcotics trafficking investigation that culminated Thursday in several arrests, police said.

Some of the seized guns have already been linked to shootings and homicides in Oakland, Anne Kirkpatrick, chief of the Oakland Police Department, said at a news conference. Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives began the investigation more than a year ago, but they could not join the news conference because of the partial government shutdown, Kirkpatrick said.

Despite working without pay, officers with the ATF and Drug Enforcement Administration were on the team that served search and arrest warrants Thursday morning, police said. Those taken into custody — Oakland officials wouldn’t say how many — were being interviewed by the afternoon. They are expected to face criminal charges in federal court.

“This morning’s very large operation is not done,” Kirkpatrick said. “We are focused on firearms and narcotics trafficking, such as the suppliers and sales, which includes fully automatic, high-powered rifles and high-capacity magazines.”

The seized guns were linked to fatal shootings in Oakland using the National Integrated Ballistic Information Network, Kirkpatrick said. The operation is ongoing, and officials are seeking to arrest additional suspects.

“If you are in possession of an illegal firearm, be concerned,” Kirkpatrick said. “We are not done.”

The operation came after a violent start to the year. Weekend shootings in East and West Oakland — including a triple shooting — killed six people. Yet city officials recorded fewer homicides in 2018 than they had in nearly two decades, and the four-year homicide average dropped to the lowest level since record-keeping began in 1985.

Elected city officials in Oakland have forbidden the police force from working with federal officials in Homeland Security Investigations, the criminal arm of U.S. Immigration and Customs Enforcement. But the City Council last month authorized other local-federal partnerships, including with the ATF and DEA. The agreements allow Oakland police officers to work in federal task forces and ensure the federal government reimburses the city for their time.